Project Summary: One in seven men in the United States will be diagnosed with prostate cancer (PCa) in his lifetime. While androgen deprivation therapy has achieved considerable success as the mainstay of PCa therapy since the 1940s, castration resistance remains common and devastating. As the third leading cause of cancer-related death in men in the United States, PCa results in over 26,000 deaths annually. Of patients who die from PCa, 90% are found to have bone metastases at autopsy. Strikingly, the five year survival rate for all PCa patients with spread to distant sites is 29%, while the same statistic for those with bone metastases is less than 3%. Any significant attempt to increase overall survival for patients with PCa must, therefore, be targeted to prevent, delay or eradicate bone metastasis. Tumor-derived exosomes have recently garnered significant attention for their potential as biomarkers, and also as mediators of pro-malignant signaling in the tumor microenvironment. Tumor derived exosomes have been implicated in a diverse set of malignant processes including the epithelial-to- mesenchymal transition, angiogenesis, local and systemic immune suppression, and metastasis. The long term goal of the proposed work is to study the role of prostate tumor-derived exosomes (PCa exos) in promoting bone metastasis, and to investigate the therapeutic potential of agents capable of attenuating exosome-mediated malignant phenotypes. Preliminary evidence indicates that PCa exos are capable of (1) robust uptake efficiency in osteoclast precursors, (2) activation of NF-kB in a monocyte reporter line, (3) stimulating osteoclast differentiation of bone marrow macrophages in vitro, and (4) trafficking to bone when systemically injected in vivo in mice. Furthermore, preliminary evidence shows that a synthetic nanoparticle-based HDL mimetic (HDL NP) is capable of inhibiting each of the four PCa exo-mediated effects stated above. Given these observations, it is hypothesized that PCa exos play a role in promoting bone metastasis by signaling to osteoclast precursors to stimulate bone resorption. Further, it is hypothesized that HDL NPs are capable of reducing PCa bone metastasis by inhibiting PCa exo-mediated signaling. In the first aim of the study, we will investigate the relationship between the osteolytic activity of PCa exos and the bone metastatic potential of their tumor cell lines of origin. We will conduct molecular and functional studies of PCa exo-induced osteolysis in vitro and in vivo. In the second aim, we will investigate the role of PCa exos in promoting bone metastasis in a clinically relevant model of castrate-resistant bone metastatic PCa, assessing the impact on bone tumor establishment and growth, and overall survival. In the third and final aim of the study, we will investigate the potential of HDL NPs as inhibitors of PCa exo-mediated signaling in bone, and as inhibitors of skeletal metastasis in a bone metastatic model of PCa. Through these studies, we hope to clarify the role of PCa exos in bone metastasis, and determine the efficacy of HDL NPs as inhibitors of PCa exo-mediated signaling.